Various embodiment of the invention relate generally to modulators and particularly to sigma delta modulators.
High-resolution analog to digital converters typically include a modulator, with the modulator, in some cases, being a sigma delta type of modulator. The reference buffer is generally a part of the feedback path (or circuit) of the sigma delta modulator. For example, a single-loop sigma-delta modulator generally includes a loop filter, a 1-bit quantizer, and a feedback circuit, where the feedback circuit has a reference buffer, an optional single-ended to differential converter, and a multiplexer. An analog input signal, “IN”, serves as the input to the modulator, a feedback signal, “FB”, serves as the feedback signal to the modulator, and the output of the modulator, “OUT”, serves as the 1-bit digital modulator output signal. The bandwidth of interest is referred to herein as “BW”.
Depending on the polarity of “OUT” (1 or −1), either voltage “+Vref_buf” or “−Vref_buf”, which are a part of the feedback circuit, is fed back to the loop filter, as one of the loop filter's input, “FB”, through the multiplexer.
The reference buffer typically includes an amplifier, an input chopper-stabilization switch, and an output chopper-stabilization switch. “Vref—in” serves as the input signal and “Vref—out” serves as the output signal of the reference buffer. The chopper-stabilization switches are toggled by a chopping signal, such as “clk_sq”, which is generally a square wave sequence (Γ[k] with “k” as an integer) running at frequency fchop. In general, frequency fchop is high enough (>BW) such that when “clk_sq” up-converts the offset (Voffset) and low-frequency (flicker) noise of the reference buffer to frequency fchop and its harmonic, the up-converted offset and flicker noise is out-of-band and no longer degrades the in-band (<BW) signal-to-noise ratio (SNR). However, the sigma-delta modulator output, “OUT[k]” with “k” as an integer, includes quantization noise with its magnitude increasing with frequency. When OUT[k] controls the feedback switch to send either “+Vref—buf” or “−Vref—buf” to signal “FB”, a mixing process occurs, and during the mixing process, low-frequency noise is undesirably generated from down-conversion of the quantization noise from OUT[k]. This low-frequency noise in signal “FB” is then fed back to the input and is passed to the output of the modulator, which degrades signal-to-noise ratio (SNR) of the output signal.
This is perhaps best understood by showing, mathematically, the effect on FB, as follows:FB=OUT[k]×Vref—buf  Eq. (1)=OUT[k]×(Vref—in+Voffset×Γ[k])  Eq. (2)=OUT[k]×Vref—in+Voffset×Γ[k]×OUT[k]  Eq. (3)=FBideal+FBnon-ideal  Eq. (4)FBideal=OUT[k]×Vref—in  Eq. (5)FBnon-ideal=Voffset×Γ[k]×OUT[k]  Eq. (6)
FBnon-ideal is the undesired mixing product of OUT[k] and chopping signal sequence Γ[k], and it contains low-frequency noise generated from down-conversion of the quantization noise from OUT[k] by Γ[k]. This low-frequency noise in signal “FB” is then fed back to the input and gets passed to the output, which degrades the SNR of the signal.
Conventionally, there are two ways to mitigate the quantization noise down-conversion, as follows:                1. Minimize Voffset. This requires larger device sizes for better random mismatch and careful layout for smaller systematic offset in the reference buffer. This leads to larger circuit area, which increases costs, and power consumption is increased.        2. Minimize fchop. The smaller the fchop, the smaller the quantization noise is to be down-converted. However, if fchop is too low, it would lead to ineffective chopper-stabilization, e.g. large residual in-band flicker noise.        Both approaches 1. and 2. above impose constraints on the design of a sigma delta modulator, and neither of them can successfully remove the quantization noise down-conversion completely.        
Accordingly, there is a need for an analog to digital converter having a modulator with precise signal measurement for improving noise performance thereof while also reducing die size and power consumption.